Conventionally, as an interior member of an automotive vehicle having an air bag door portion such as an instrument panel, a door trim, a center pillar and the like, there has been known a structure described in Japanese Patent Application Laid-Open (JP-A) No. 8-192666.
In the instrument panel having the air bag door portion disclosed in the above publication, a main body portion of the instrument panel and the air bag door portion are integrally formed by so-called dual-injection molding (double injection molding) wherein the air bag door portion (an opening portion) is injection molded with a thermoplastic elastomer, after a main body portion of the instrument panel having an opening portion for the air bag door is injection molded with a thermoplastic resin. Further, as a popularly known structure, there is an instrument panel to which an independent air bag door (injection molded with a thermoplastic elastomer) is later attached.
However, in the instrument panel having the air bag door portion mentioned above, in a case wherein the whole of the instrument panel is constructed of the same resin without replacing the resin of the air bag door portion with the resin of the main body portion, on the basis of characteristics of the resin in the main body portion, the rate of elasticity of the resin in the main body portion is 6 to 7 times higher than the rate of elasticity of the resin (TPO) in the air bag door portion, and the tensile strength in the main body portion is 1.5 to 2 times higher than the tensile strength in the air bag door portion; thus, the break force of a portion for expansion formed in the air bag door portion, an H-shaped break portion (a tear portion), for example, becomes higher in a case of the same thickness, so that it is hard for the air bag door portion to expand, and moreover, a hinge portion may break during expansion. If in order to solve this problem, the thickness of the resin in the break portion is made too thin, underfill, oil-can-like-feeling and deformation are generated. Particularly, in a hard resin instrument panel, since unevenness appears in a periphery of the thin portion at an outer appearance side due to weld shrinkage, an undulation and the like, the quality of the appearance of outer is lowered.
Further, in order to achieve a state wherein both a line of the break portion and an outer periphery of the air bag door portion are completely invisible from the outer appearance side (an invisible type), there is a method of making the break portion thick during molding and then cutting the break portion through a later process. However, in this method, it is very difficult to make the line in the break portion invisible while maintaining a predetermined break force. That is, when the thickness of the resin is made thin, the line and the undulation in the break portion are seen from the outer appearance side even in a case wherein a relief groove is provided, so that the quality of the outer appearance can not be maintained, and the thickness of the resin can not be made sufficiently thin.
When it is difficult to expand the door portion due to the increased break force required during expansion at the break portion (tear portion), which is formed at the air bag door portion, and in order to ameliorate this the thickness of the resin in the break portion is made too thin, the thin portion becomes visible from an outer side and so the quality of the exterior appearance deteriorates; this deficiency is also generated with respect to: a break portion of an interior member of an automotive vehicle (vehicle interior member) such as an instrument panel, door trim, center pillar, or the like, wherein the interior member is integrally formed in accordance with dual-injection molding, using different resins for the main body portion of the vehicle interior member and for the air bag portion; and a break portion of a vehicle interior member wherein the vehicle interior member is obtained by integrally assembling the air bag door portion and the main body portion of the instrument panel by means of a locking hook, a screw or the like, after independently molding the air bag door portion and the main body portion of the instrument panel.
As for technologies relating to the present invention, there are: Japanese Patent Application Laid-Open (JP-A) Nos. 3-281457 and 7-179161 in which stress is concentrated at a foaming layer and a skin in a door-integral foaming instrument panel during an initial period of expansion, Japanese Patent Application Laid-Open (JP-A) No. 2-147452 in which a bead is formed along a tear portion in a later attached resin door, Japanese Patent Application Laid-Open (JP-A) No. 5-185898 in which a blade is set at a distal end of a door metal plate insert in a door-integral foaming instrument panel, Japanese Patent Application Laid-Open (JP-A) No. 7-291078 in which a hinge portion is reinforced in a hard instrument panel obtained by integrally molding a door portion and a main body of the instrument panel, Japanese Patent Application Laid-Open (JP-A) No. 8-290749 in which a hinge portion is reinforced in a skin instrument panel obtained by integrally molding a door base member and an instrument panel base member, Japanese Patent Application Laid-Open (JP-A) No. 2-283546 in which a rib applying maximum shearing force to a tear portion of an independent door is integrally formed, Japanese Patent Application Laid-Open (JP-A) No. 2-109848 in which at a can lid, a thin line is formed in a resin layer so as to surround an opened portion and a weld line is set on the thin line, Japanese Patent Application Laid-Open (JP-A) No. 7-291078 in which a reinforcing member is thermally caulked on a reverse face of a door portion in a resin instrument panel and a notch is formed on a tear line of the door portion, and Japanese Patent Application Laid-Open (JP-A) No. 8-20040 in which a different kind of resin is injected by shielding a metal mold cavity by means of a shielding core so as to perform a core-back immediately before the injection is completed.